In gas turbine engines air passes from the compressor through a pressure recovery diffuser enroute to the combustor. The diffuser case has an outer case and an inner case forming an annular flowpath between them. Since they surround the rotor shaft, the inner case must be supported from the outer case. Sometimes struts are used between the two case sections with this introducing weight and interference with the gas flow.
Cambered airfoil vanes are used at the upstream end of the diffuser to straighten rotating airflow leaving the compressor. These are connected to an upstream extending shroud on both the outer and inner cases. They offer opportunity for support provided reasonable stress levels can be achieved.
Complex loading is imposed on such vanes, however. The inner case imposes a high axial load towards the downstream end, imposing high axial loading through the vanes. A circumferential force is imposed on the vanes because of the differential pressure between the pressure and suction side of each vane. There furthermore is a high torque loading caused by rotational forces on the inner case. This causes a circumferential shear loading on the vanes.
The high axial load is substantially in the plane of the vanes. The shroud case can be made stiff to take this load without excessive stress. The side loading caused by the pressure on the vanes is relatively insignificant and is added to the circumferential load. The circumferential load or torque load causes the vane to resist the loading as a fixed end beam in shear. Therefore, high bending stresses exist at the end points. Decreasing the shroud thickness would permit rotation at the end of the vanes thereby decreasing the bending stresses, but in such a case the axial load could not be tolerated by the reduced thickness shroud.
The vanes are not flat, but are cambered airfoils. High stress, therefore is found not uniformly along the vanes, but at locations on the order of 25 to 30 percent from each end.